The present invention relates to the field of electronic article surveillance (EAS) systems, and in particular, to EAS antenna systems for both transmitting and receiving signals in all directions.
Antenna systems usually comprise two units positioned either side of a store doorway or at a checkpoint aisle, and are often referred to as gates, pedestals, panels, or detectors. In many systems, one antenna fulfills the role of transmitter, with the other acting as receiver. Single antenna systems may also be offered which combine transmitter and receiver functions into a single transceiver unit. This may be advantageous where space is limited.
Antennas at the doorway are coupled to a control system. In response to the control system, the antenna emits an energy field or frequency signal into a surveillance area. An anti-theft tag or marker that enters the antennas"" surveillance area responds to this energy field by emitting its own frequency signal. When the antennas pick up this return frequency, an audible alarm is triggered on the antennas.
The arrangement and operation of an antenna system contributes to ensuring reliable and efficient performance of an electronic article surveillance (EAS) system. Antenna systems used in EAS sometimes do not detect markers. As an example, certain positions and orientations of a marker in a surveillance area can be optimal, whereas other positions and orientations can be inferior. Likewise, it is possible for a marker to be positioned in an orientation wherein it is sufficiently excited by an antenna and not be simultaneously in an orientation that the marker can be optimally detected or received by the antenna.
In the case of systems like the Ultra-Max(copyright) of Sensormatic Electronics Corporation, a first rectangular upper antenna loop and a rectangular second lower antenna loop are positioned in an in-phase mode to transmit signals into a surveillance area. A set of receiver antennas are positioned to operate, which also consists of one upper antenna and one lower antenna, receives signals from the surveillance area.
A disadvantage of this arrangement is the strength generated by the magnetic field in the vertical direction and middle position can be inadequate for reliable detection of a marker in the surveillance area. This is theorized to be due to rectangular loop or in-phase mode antenna configuration of the Ultra-Max(copyright) system, which provides substantial horizontal magnetic field, but a substantially weaker vertical magnetic field.
An improved prior art is Sensormatic""s Ultra-Post system as disclosed in U.S. Pat. No. 5,963,173, wherein a first upper antenna and a second lower antenna work in interlaced modes, or in-phase transmission mode and out-of-phase transmission mode. Out-of-phase mode simply means the loops are driven by current flowing in opposite directions. The system generates substantial magnetic field in a vertical direction, but little or no strength in a horizontal direction. A receiver adds up the signals detected from both a first upper receiving antenna and a second lower receiving antenna. Signals received by the two receiving antennas may be added together to form a composite signal depending on whether signals are 90 degrees out of phase or 180 degrees out of phase.
The inter-laced mode of operation transmits signals in both horizontal and vertical directions. The in-phase mode provides significant coverage in a horizontal direction while the out-of-phase mode provides significant coverage in a vertical direction. The two modes of operation compliment each other.
With the Ultra-Post system, the different transmission modes make it difficult to tune antenna sets for accurate resonant frequency. Therefore, some additional compensation circuit is used to compensate for this design. Secondly, transmitting antennas and receiving antennas are separately dealt with in the Ultra-Post system instead of being treated as a whole system to optimize overall system performance.
What is needed is an improved system and method wherein a transmitting and receiving antenna are arranged as an integrated system to provide better magnetic field coverage in all directions without requiring multi-source signals, which are different in phase.
An antenna system for an electronic surveillance system, in accordance with an inventive arrangement, comprises a first antenna and a second antenna, the first antenna and the second antenna mounted for use in a substantially overlapping manner, the first antenna configured as a loop, the second antenna configured in a xe2x80x9cfigure-8xe2x80x9d; and, a control circuit for alternatively activating the first and second antennas.
The first and second antennas can each operate one at a time as a transmit and receive antenna. The first antenna and second antenna substantially lie in a common plane. Thus, by alternatively toggling between operation of the first antenna and the second antenna all markers in the surveillance area can be detected regardless of position and orientation. Furthermore, the system is configured that the first antenna transmits and then receives while the second antenna is open circuit. Alternatively, the second antenna transmits and then receives while the first antenna is open circuit. In one case, while the second antenna is open circuit, the first antenna substantially detects a marker entering into a surveillance area, the marker oriented substantially perpendicular to an elongated axis with respect to the first antenna. Alternatively, while the first antenna is open circuit, the second antenna substantially detects a marker entering into a surveillance area, the marker oriented substantially parallel to an elongated axis with respect to the second antenna.
The control circuit further comprises a switch to selectively enable operation of the first antenna while the second antenna is open circuit. Likewise, the control circuit further comprises a switch to selectively enable operation of the second antenna while the first antenna is open circuit. An optional ferrite rod can be used for intensifying a magnetic field in each of the first and second antennas.
The system preferably includes a receive circuit and a transmit circuit coupled to each of the first and second antennas for receiving and transmitting signals. Preferably, a single receive circuit and transmit circuit are alternatively coupled to the first and second antennas. The receive circuit receives signals from a surveillance area while the transmit circuit is open circuit. Alternatively, the transmit circuit transmits signals into a surveillance area while the receive circuit is open circuit.
A method of arranging an antenna system for an electronic article surveillance system, in accordance with another inventive arrangement, having a first antenna and a second antenna, the first antenna configured as a loop, the second antenna configured in a xe2x80x9cfigure-8xe2x80x9d, comprises the steps of: mounting the first antenna and the second antenna in a substantially overlapping manner; and, field-coupling a control circuit to the first and second antennas for alternatively activating the first and second antennas.
The first and second antennas can each operate as a transmit and receive antenna. Furthermore, the first antenna can transmit and then receive while the second antenna is open circuit. Alternatively, the second antenna can transmit and then receive while the first antenna is open circuit. In all cases, each of the first and second antennas can detect a marker entering into a surveillance area. In one case, while the second antenna is open circuit, the first antenna substantially detects a marker entering into a surveillance area, the marker oriented in a position substantially perpendicular to an elongated axis with respect to the first antenna. Alternatively, while the first antenna is open circuit, the second antenna substantially detects a marker entering into a surveillance area, the marker oriented in a position substantially parallel to an elongated axis with respect to the second antenna.
The control circuit comprises a switch to selectively enable operation of the first antenna while the second antenna is open circuit. Alternatively, the control circuit further comprises a switch to selectively enable operation of the second antenna while the first antenna is open circuit. A ferrite rod can be used for intensifying a magnetic field in each of the first and second antennas.
The method can further include a receive circuit and a transmit circuit coupled to each of the first and second antennas for receiving and transmitting signals. The receive circuit receives signals from a surveillance area while the transmit circuit is open circuit. Alternatively, the transmit circuit transmits signals into a surveillance area while the receive circuit is open circuit.